1. Field of the Invention
This invention relates to a magnetic field sensor using an MTJ junction cell. In particular, it relates to the use of such a magnetic field sensor also as a current sensor and to a method of protecting the sensor from electrostatic discharge (ESD) by means of a shunt.
2. Description of the Related Art
Basic Operation of GMR and MTJ Cells
Two types of small, multilayered magnetic devices that change their resistance in response to variations in an external magnetic field are presently in wide use in technologies such as read heads in hard disk drives and magnetic information storage devices or MRAM. These two device types are the GMR, or giant magnetoresistive device and the MTJ or magnetic tunneling junction device. The devices are similar in that both include a pair of thin, magnetized ferromagnetic layers separated by a nonmagnetic layer. In the case of the GMR device, the nonmagnetic layer is a conductor, such as copper. In the case of the MTJ device, the nonmagnetic layer is a dielectric, such as aluminum oxide. The physical basis behind the operation of the GMR device is that when the two ferromagnetic layers are magnetized in different directions, the electrons in a current passing through one ferromagnetic layer towards the other ferromagnetic layer, through the intervening copper layer, are scattered differently in the various material layers and at their interfaces according to the directions of their spins relative to the directions of the magnetic moments. This spin-dependent scattering makes the GMR device act like a variable resistor.
The variable resistance of the MTJ device, on the other hand, does not result from spin-dependent scattering, but from spin-dependent tunneling. The layer between the two magnetized ferromagnetic layers in the MTJ device is not a conductor, but is a dielectric of such thinness that electrons nevertheless have a probability of tunneling through it from one ferromagnetic layer to the other. This probability, however, depends of the spin direction of the tunneling electron relative to the magnetic moment of the ferromagnetic layer towards which it is tunneling. When the electrons first pass through a reference layer of ferromagnetic material that is magnetized in one direction, their normally randomized spin directions are preferentially aligned with this magnetization. Then, their probability of tunneling through the intervening dielectric “tunneling barrier” layer depends of the magnetization direction of the ferromagnetic layer towards which they are proceeding. Since the present invention will deal with MTJ type devices, their structure will be described in some further detail below with reference to FIG. 3.
Basic Operation of a Current Sensor
Referring now to FIG. 1A, there is shown a schematic overhead illustration of how a prior art structure containing patterned MTJ cells has already been used to measure a current passing through a conductor of particular shape by measuring the strength of its magnetic field at particular places. This particular prior art structure is shown in related patent application U.S. Ser. No. 11/788,912, Filing Date Apr. 23, 2007, that is fully incorporated herein by reference. Shoji, in US Published Patent Applications (2006/0071655) and (2006/0170529) teaches the use of unpatterned GMR cells to measure the current in conductors of various shapes.
According to the illustration in this figure, Im, a current to be measured (signified by a large open arrow), enters into an exemplary horseshoe-shaped conductor (80) at a contact denoted “Pad 1” (83) and leaves at Pad 2 (85). Two MTJ current-sensing structures, (1000), (2000), are positioned adjacent to the conductor as shown. For exemplary purposes only, each of these structures is a configuration of four (other numbers being possible) identical elliptically patterned MTJ cells, (100), connected electrically in parallel by a pair of electrodes, one electrode contacting the tops of the cells, and the other electrode contacting their bases. Only the top electrode (500), in each structure can be seen in this overhead view, a side view in FIG. 1B will clarify the electrode positioning.
Each sensor structure (1000) and (2000) is supplied with its own current, Is, which enters and leaves through the electrodes. This current, which will be shown clearly in FIG. 1B, is for sensing purposes and will allow the variations in MTJ cell resistance to be converted into measurable variations in voltage across the cells. If the sensing current, Is, is fixed, the measurable voltages will be related directly to the angle between the magnetic moments of the ferromagnetic layers. For sensing and measuring the strengths of external magnetic fields, the individual cells might be initially magnetized so that one ferromagnetic layer (the “pinned” layer) has its magnetic moment fixed in direction while the other ferromagnetic layer's magnetic moment is free to move (the “free” layer). The magnetic moments of the free and pinned layers are perpendicular to each other, with the pinned direction being along the shorter axis of the elliptical cell and the free direction being along the long axis. With these magnetization directions, the direction of the external magnetic field (arrows (150) and (160)) induced by the current, Im, in the conductor can most effectively vary the direction of the free layer's magnetic moment and the amount of variation would be indicated by the voltage drop across the cell.
By placing the cells alongside the two opposite sides of the horseshoe shaped conductor as shown in the figure, they experience the induced magnetic field of Im in opposite directions as shown by the two sets of magnetic field arrows (150), (160) (which are directed along the short axes of the elliptical cells and, therefore, are perpendicular to the free layer magnetization). Then, a differential amplifier (not shown) measures the difference between voltage drops. The difference signal produced by the differential amplifier eliminates random temperature-induced noise fluctuations from the cells and any effects of stray external magnetic fields because these fluctuations are cancelled out, but the oppositely directed magnetic fields cause the current produced voltage drops to be of opposite value and, therefore, to add. It is noted that the two sensor structures (1000) and (2000) may be formed in physical contact with the conductor (80), but not in electrical contact.
Referring to FIG. 1B, one of the structures of FIG. 1A, either (1000) or (2000) is illustrated in a schematic side view. The top and bottom electrodes are now distinguishable as (500) and (600) and the same four cells, (100), are now shown as multilayered devices in a side cross-sectional view. The cells would be separated from each other by a dielectric material (700) that was deposited after the cells were formed on the lower electrode and then patterned and planarized to provide a smooth planar surface for contacting the MTJ cell upper surface with the top electrode. Although it is not indicated in the figure, the bottom electrode may be in physical contact with the conductor ((80) in FIG. 1A) but separated from it by an insulating layer.
If a current, Is, (shown as entering arrow (111)) is injected into top electrode (500) and extracted from bottom electrode (600) (also shown as exiting arrow (111)), it will pass through the cells (100), as shown by the downward directed arrows (111). If the resistances of these cells are substantially equal, as would be the case if the angles between their free and pinned magnetic moments are the same, each cell will experience the same voltage drop and an equal current will pass through each of them.
Protecting the Sensor From Electrostatic Discharge (ESD)
The MTJ junctions of MTJ cells normally have a very low breakdown voltage (less than 2 volts) and are highly susceptible to damage from electrostatic discharge (ESD) during handling, packaging and assembly. When an MTJ cell is exposed to ESD, the junction may be totally or partially damaged and the sensing device of which it is a part will malfunction.
The prior art discloses several methods to protect MTJ cells from ESD damage. Jayasekara et al. (US Published Patent Application 2007/0076328) forms a shunt out of ferromagnetic material to protect an MTJ cell in a hard disk drive. Granstrom et al. (U.S. Pat. No. 7,119,995) discloses an ESD shunt designed to protect a MTJ read head during fabrication. The shunt is removed after fabrication.
Both of the shunts taught above are directed at single MTJ read heads to be used in a hard disk drive environment. MTJ cells to be used as current sensors operate in a different environment and are configured differently. What is needed, therefore, is a mechanism to protect MTJ cells in a current sensor configuration from the effects of ESD.